1. Field of the Invention
The present invention relates to a semiconductor light emitting device used in an optoelectronic integrated circuit and an image display device, and more particularly to a semiconductor light emitting device and a method for manufacturing the same using a porous silicon.
2. Description the Related Art
The porous silicon (hereinafter referred to as a xe2x80x9cPSxe2x80x9d) differs from a crystalline silicon (hereinafter referred to as a xe2x80x9cc-Sixe2x80x9d) in optical properties, and absorption edge energy generally becomes large. Moreover, electrical. properties also changes, and the resistivity becomes high as compared with the original c-Si. Three kinds of PSs are known as follows:
(a) NANOSTRUCTURE PS:
The PS of which a porosity is 20 to 80% and the diameter of microporous holes is not more than approximate 2 nm is referred to as xe2x80x9ca nanostructure PSxe2x80x9d. Differing from the c-Si, the nanostructure PS shows luminescence in a visible-light range. Pumping this nanostructure PS by the shorter wavelength light within the spectral region from blue to ultra violet, photoluminescence (PL) of a luminescence efficiency (external quantum efficiency) of approximate 10% at the maximum can be observed. Moreover, electroluminescence (EL) can be obtained also by injecting current into the nanostructure PS.
(b) MESOSTRUCTURE PS:
On the other hand, the PS of which the porosity is 40 to 60% and the diameter of the microporous holes is approximate 2 to 50 nm is referred to as xe2x80x9ca mesostructure PSxe2x80x9d. The luminescence efficiency of the mesostructure PS is generally low as compared with the nanostructure PS, and an emission wavelength also generally comes to the longer wavelength than the nanostructure PS. The mesostructure PS is coarse in structure as compared with the nanostructure PS and is low in resistivity as compared with the nanostructure PS.
(c) MACROSTRUCTURE PS:
Moreover, a PS of which the porosity is further low than the mesostructure PS and the diameter of the microporous holes is 50 nm or more is referred to as xe2x80x9ca microstructure PSxe2x80x9d. The macrostructure PS can hardly emit light and is further low in resistivity as compared with the mesostructure PS.
These PSs are formed by anodization, or by feeding a current inwardly from the surface of the silicon through the c-Si (single crystal silicon or polycrystalline silicon) as electrodes in the solution containing hydrogen fluoride (HF). Moreover, as a cathode, materials such as platinum (Pt) being usually not dissolved into an anodization solution is used. Although the PS and the material having the structure similar thereto can be made by other methods, they are omitted because of being not important in the invention.
Thus, the PS is constituted by the number of the microporous holes of the diameter of approximate 1 to 100 nm, remained small c-Si particles or a skeleton, and an amorphous portion surrounding thereabouts. By changing the conditions such as the conductivity type and resistivity of the original c-Si, the current density at anodizing, the composition of the anodization solution, the presence or absence of light irradiation and the intensity of the light irradiation, the structure of the PS being made is changed, whereby the nanostructure PS, the mesostructure PS or the macrostructure PS can be obtained.
For example, the nanostructure PS is obtained by anodizing the c-Si containing a p-type impurity doped to the extent being not degenerated (a non-degenerate p-type). Moreover, the nanostructure PS is obtained also by anodization while irradiating a non-degenerate n-type c-Si of a low impurity concentration with light. This nanostructure PS is fine so that the porosity is approximate 20 to 80% and the diameter of the holes is not more than 2 nm. That is, since remaining c-Si particles or a size of the skeleton are fine, the resistivity becomes high as compared with that of the original c-Si. For example, the nanostructure PS can be obtained by anodizing the degenerate p-type c-Si or the degenerate n-type c-Si, containing the p-type or n-type impurity with higher impurity concentration so that the Fermi level is located within the valence or conduction band. For example, the macrostructure PS can be obtained by anodizing the non-degenerate n-type c-Si in a darkroom.
The above-noted description of the three kinds of PSs which differ in structure is performed on the generalized characteristics of the respectively typical one, and actually, there are the PSs having the characteristic intermediate between the mesostructure PS and the nanostructure PS and the PSs having the characteristic intermediate between the mesostructure PS and the macrostructure PS or the like. Moreover, for example, even the PS belonging to the same nanostructure PS can differ in the fine structure in some cases depending upon the difference of a conductivity type of the original c-Si. Moreover, even though the original c-Si is uniform, the PS of which the structure differs in the direction of a depth can be made depending upon anodizing conditions. Furthermore, even the PS belonging to the mesostructure PS or the macrostructure PS as the general structure and characteristic, the PS containing the nanostructure PS can be made partially in the microscopic portion depending upon the anodizing conditions.
Therefore, when making a light emitting device using the PS, a sufficient consideration should be taken in the both sides of the element design from the viewpoint of by which structure PS a layer is constituted and for what it is used, and a selection of a method for making the element structure.
It is reported in the proceedings of the 44 th Japan Society of Applied Physics and Related Society Symposium, No.2, P.806, Section a-B-6, xe2x80x9cCharacteristics of a pn-junction type photoanodically fabricated porous silicon LEDxe2x80x9d, by Nishimura, Nagao and Ikeda that external quantum efficiency of the EL luminescence comes to approximate 1% at the maximum in the light emitting device using the PS (hereinafter referred to as a xe2x80x9cPS light emitting devicexe2x80x9d). This PS light emitting device is made by preparing a c-Si wafer that the p+ type c-Si layer is formed on the n-type c-Si substrate to anodize the surface of this c-Si wafer under the irradiating with light using a lamp. When anodizing under such conditions, the p+ type c-Si layer of the surface of which resistivity is low becomes the mesostructure PS layer and the n-type c-Si substrate portion of the area which no light from the lamp reaches becomes the macrostructure PS. In FIG. 1 and FIG. 2, the structure and the equipment for manufacturing this PS light emitting device are shown.
Referring to FIG. 1, the macrostructure PS layer 63 made from the n-type c-Si, hereinafter referred to as xe2x80x9ca n-type macrostructure PS layer 63xe2x80x9d, is formed on a n-type c-Si substrate 64. And the nanostructure PS layer 62 made from the n-type c-Si, hereinafter referred to as xe2x80x9ca n-type nanostructure PS layer 62xe2x80x9d is formed on the n-type macrostructure PS layer 63. And further the mesostructure PS layer 61 made from the p+ type c-Si, hereinafter referred to as a p-type mesostructure PS layer, is formed thereon. Moreover, the expressions of xe2x80x9cthe n-type macrostructure PS layerxe2x80x9d, xe2x80x9cthe n-type nanostructure PS layerxe2x80x9d, xe2x80x9cthe p-type mesostructure PS layerxe2x80x9d or the like are expressed for convenience and differ from the n-type and the p-type in the c-Si. The reason why is that, generally, in the PS layer, acceptor impurities and donor impurities are inactivated at room temperature. A translucent gold electrode 66 which serves as an anode is formed on the p-type mesostructure PS layer 61 and an aluminum electrode 65 which serves as a cathode is formed on the back of the n-type c-Si substrate 64. A direct current power supply 67 for the EL is connected between the anode 66 and the cathode 65. In the structure shown in FIG. 1, the n-type nanostructure PS layer 62 acts as an EL active layer. Moreover, the p-type mesostructure PS layer 61 has a function to form a junction similar to the pn-junction in the c-Si hereinafter, such kind of junction by the PS layer is referred to as a xe2x80x9cthe pn-junctionxe2x80x9d for convenience) between the n-type nanostructure PS layer 62 and the p-type mesostructure PS layer 61, and a function to get better ohmic contact with the translucent gold electrode 66 formed on the layer 61.
In order to form the structure shown in FIG. 1, first, a c-Si wafer 7 on which a p+ type c-Si layer 71 of 0.6 xcexcm in thickness and resistivity of 2xc3x971031 3 xcexa9-cm is formed on a surface of a n-type c-Si substrate 72 of 500 xcexcm and resistivity of 5 xcexa8-cm using a thermal diffusion method is prepared. Subsequently, it can be manufactured by anodizing this c-Si wafer 7 as shown in FIG. 2. That is, as shown in FIG. 2, a container 1 for anodization, which is made of polytetrafluoroethylene(PTFE), having an opening on the bottom is contacted closely with the surface of the p+ type c-Si layer 71 using O-rings 2 to fill an anodizing mixed solution consisting of hydrofluoric acid and ethyl alcohol 4 into this container made of PTFE 1. Because of using the O-ring 2, the anodizing mixed solution consisting of hydrofluoric acid and ethyl alcohol 4 does not leak from the bottom of the container 1 made of PTFE. The anodization solution 4 consists of hydrofluoric acid of 50 weight percent and ethyl alcohol of 99.9 weight percent mixed at a volume ratio of 1:1. In the anodizing mixed solution consisting of hydrofluoric acid and ethyl alcohol 4, a platinum electrode 3 is arranged. On the other hand, on the back of the n-type c-Si substrate 72, the aluminum electrode 65 which will become the cathode shown in FIG. 1 eventually, and a desired anodizing current is fed through the anodizing mixed solution consisting of hydrofluoric acid and ethyl alcohol 4 by the variable direct current power supply 6 connected between the platinum electrode 3 and the aluminum electrode 65. The anodizing is performed while irradiating the p+ type c-Si layer 71 and the n-type c-Si substrate 72 thereunder by a tungsten lamp 5 arranged on the upper of the container made of PTFE 1. Therefore, the platinum electrode 3 is arranged such that the light radiated from the tungsten lamp 5 can not be impeded to reach the surface of the c-Si wafer 7.
The mesostructure PS layer 61 is obtained by anodizing the p+ type c-Si layer 71 shown in FIG. 1. Moreover, the nanostructure PS layer 62 shown in FIG. 1 is formed at the portion in proximity to the surface influenced by light radiation in the n-type c-Si substrate 72 shown in FIG. 2. Moreover, the macrostructure PS layer 63 is formed at a slightly inner portion from the surface not influenced by light radiation in the n-type c-Si substrate 72. The n-type c-Si substrate 64 shown in FIG. 1 is a portion remaining as the c-Si of the n-type c-Si substrate 72 shown in FIG. 2. According to the method shown in FIG. 2, the longer an anodization time is, the thicker the nanostructure PS layer 62 becomes, and moreover, the macrostructure PS layer 63 formed thereunder comes to be thick increasingly in response thereto. Moreover, the anode 66 shown in FIG. 1 is the translucent gold electrode formed by evaporating a gold thin film by a vacuum evaporation method after anodizing.
The luminescence efficiency (the quantum efficiency) depends upon a way of anodizing and the anodization time, thereby not always being constant. Generally, the nanostructure PS layer anodized sufficiently by extending the anodization time has higher luminescence efficiency than that of the nanostructure PS layer anodized insufficiently with the shorter anodization time.
To some extent, the longer the anodization time becomes, the higher the external quantum efficiency and higher the electric power efficiency of the PS light emitting device become. The reason why is that when the anodization time is made long, so that anodizing is promoted sufficiently, the quantum efficiency increases. However, when the anodization time is long excessively, the electric power efficiency decreases again. This reason why is that, although the external quantum efficiency becomes higher owing to the increase of the thickness of the nanostructure PS layer 62 being the light emitting layer in company with the increase of the anodization time, the increase of the series resistance of the nanostructure PS layer whose resistivity is high becomes prominent when exceeding a certain thickness. This is to be understood by referring to FIG. 3. That is, FIG. 3 shows a relationship between a series resistance Rs of such PS light emitting device and a thickness xe2x80x9cdxe2x80x9d of the nanostructure PS layer 62. Referring to FIG. 3, a symbol of  shows the series resistance Rs of the light emitting device having the n-type nanostructure PS layer 62 shown in FIG. 1. And a symbol of  shows the series resistance Rs of the light emitting device, in which a luminescence layer is constituted by the p-type nanostructure PS layer made from the p-type c-Si, having the approximately same element structure as that shown by the symbol of . It is understood that there is a relationship of approximately Rs∞d2xcx9c3. Therefore, for the light emitting device whose external quantum efficiency is high, the series resistance thereof becomes high inevitably. Especially, the series resistance Rs of the PS light emitting device, whose external quantum efficiency is high as 0.1 to 1%, becomes high as 100 k xcexa9 to 1M xcexa9, and a high supply voltage is required in order to inject a current into such PS light emitting device. That is, electric energy converted to thermal energy is more increased with respect to electric energy converted to light energy, whereby the electric power efficiency would be decreased.
The present invention is devised for solving the problems of the prior art described above, and the object of the invention is to reduce a series resistance Rs of a PS light emitting device to improve an electric power efficiency without impairing external quantum efficiency.
The further object of the invention is to provide a light emitting device of which an operation voltage is low and an external quantum efficiency is high.
The another object of the invention is to provide a method for manufacturing a PS light emitting device of which control of a film thickness of the luminescence layer is easy and the external quantum efficiency and an electric power efficiency are high.
The additional object of the invention is to provide a method for manufacturing the PS light emitting device which can be integrated readily on the same silicon substrate with other electronic devices and can be manufactured inexpensively.
To accomplish the object described above, a first feature of the invention is a semiconductor light emitting device at least comprising a first one conductivity type nanostructure porous silicon (PS) layer, an opposite conductivity type mesostructure PS layer disposed on the first one conductivity type nanostructure porous silicon (PS) layer, and a first one conductivity type mesostructure PS layer formed under the first one conductivity type nanostructure PS layer. Where xe2x80x9cthe one conductivity type nanostructure PS layerxe2x80x9d is an abbreviated expression of the nanostructure PS layer formed from the one conductivity type crystalline silicon (c-Si ) and xe2x80x9cthe opposite conductivity type mesostructure PS layerxe2x80x9d is an abbreviated expression of the mesostructure PS layer formed from the opposite conductivity type c-Si. Here, if the one conductivity type is n-type, the opposite conductivity type is p-type. And, if the one conductivity type is p-type, the opposite conductivity type is n-type. Moreover, xe2x80x9cthe one conductivity type mesostructure PS layerxe2x80x9d is an abbreviated expression of the mesostructure PS layer formed from the one conductivity type c-Si. Although the one conductivity type nanostructure PS layer is the layer which functions as the main light emitting layer, the resistivity is high. On the other hand, the luminescence efficiency and the resistivity of the first one conductivity type mesostructure PS layer are low. xe2x80x9cThe nanostructure PS layerxe2x80x9d implies the PS layer of which the porosity is 20 to 80% and the diameter of microporous holes is not more than approximate 2 nm as described above. On the other hand, xe2x80x9cthe mesostructure PS layerxe2x80x9d implies the PS layer of which the porosity is 40 to 60% and the diameter of microporous holes is not more than approximate 2 to 60 nm as described above. The pn-junction is formed between the first one conductivity type nanostructure PS layer and the opposite conductivity type mesostructure PS layer, and carriers are injected from the opposite conductivity type mesostructure PS layer to the first one conductivity type nanostructure PS layer, thereby light being emitted.
The first one conductivity type nanostructure PS layer according to the invention may be formed by anodizing the non-degenerate crystalline silicon (c-Si) layer whose impurity concentration is low. That is, when anodizing the structure that the one conductivity type non-degenerate c-Si layer is sandwiched between the opposite conductivity type degenerate c-Si layer whose impurity concentration is high and the first one conductivity type degenerate c-Si layer, only the one conductivity type degenerate c-Si layer becomes the nanostructure PS layer, whereby a thickness can be controlled correctly. That is, according to a first feature of the invention, the thickness of the first one conductivity type nanostructure PS layer which serves as the light emitting layer can be controlled into the predetermined thickness that the series resistance Rs is not increased and the maximum luminescence efficiency can be obtained. Conversely, the thickness of the eventual first one conductivity type nanostructure PS layer is limited within the predetermined thickness, whereby, at anodizing, the sufficient anodization time can be expended to anodize sufficiently. That is to say, the nanostructure PS layer in the first feature of the invention is in the state that xe2x80x9ctransformation of crystalline silicon to porous siliconxe2x80x9d has promoted sufficiently, and so to speak, the layer is the completed nanostructure PS layer. Therefore, according to the first feature of the invention, the light emission from this completed nanostructure PS layer is utilized, whereby the luminescence efficiency (the quantum efficiency) is extremely high as compared with the uncompleted nanostructure PS layer in the prior art. In the first feature of the invention, it is preferable to further comprise at least a second one conductivity type nanostructure PS layer and a second one conductivity type mesostructure PS layer under this second one conductivity type nanostructure PS layer on the lower of the first one conductivity type mesostructure PS layer. xe2x80x9cThe second one conductivity typexe2x80x9d is the same conductivity type as the first one conductivity type. The second one conductivity type nanostructure PS layer also is the completed nanostructure PS layer. As is described using FIG. 3, since the series resistance Rs of the nanostructure PS layer is proportional to the square or the cube of the thickness xe2x80x9cdxe2x80x9d of the nanostructure PS layer, the series resistance becomes low suddenly when thinning the thickness of the nanostructure PS layer. Therefore, when connecting a plurality of thin nanostructure PS layers (N layers, defining N as a positive integer) in series, the total series resistance Rs (the total) becomes small value.
For example, the n-type nanostructure PS layers having the thickness of a fraction of N of the nanostructure PS layer of a single layer are prepared by N layers to form a stacked structure that the n-type mesostructure PS layers of Nxe2x88x921 layers are sandwiched between these N layers. In this case, since the resistivity of the mesostructure PS layer is extremely low as compared with that of the nanostructure PS layer, it hardly contributes to the series resistance. Therefore, the total series resistance Rs(total) of this stacked structure is reduced to a fraction of N as compared with that of the single layer of the n-type nanostructure PS. That is, when investigating the thickness of each layer of the nanostructure PS layer divided into the multi-layer such that the thickness of the total of N layers becomes the same as the thickness of the nanostructure PS layer of a single layer, light emitting intensity is approximately equal since both the thickness are equal as a whole, but the series resistance is reduced drastically due to the stacked structure. Therefore, the increase in efficiency of the electric power efficiency can be attained by the reduced series resistance. Moreover, the light emitting can be performed by a lower operating voltage.
A second feature of the invention relates to a method for manufacturing the semiconductor light emitting device according to the described-above first feature. That is, the second feature of the invention is a method for manufacturing the semiconductor light emitting device comprising the steps of: at least preparing a c-Si wafer comprising: at least the first one conductivity type degenerate crystalline silicon (c-Si) layer; the first one conductivity type non-degenerate c-Si layer formed on the first one conductivity type degenerate c-Si layer; and the opposite conductivity type degenerate c-Si layer formed on the first one conductivity type non-degenerate c-Si layer, and anodizing this c-Si wafer to transform the first one conductivity type non-degenerate c-Si layer to the first one conductivity type nanostructure PS layer. In this case, since the first one conductivity type degenerate c-Si layer and the opposite conductivity type degenerate c-Si layer are the c-Si layers whose impurity concentration are high and are transformed to the mesostructure PS layers respectively by anodization. Since the thickness of the first one conductivity type non-degenerate c-Si layer, whose impurity concentration is low, is defined correctly by an epitaxal growth method or the like, the thickness of the first one conductivity type non-degenerate c-Si layer is transformed to the thickness of the first one conductivity type nanostructure PS layer automatically and exactly, and can not be made thicker than this thickness, whereby a thickness can be controlled correctly. A degenerate c-Si substrate may be used as the first one conductivity type degenerate c-Si layer. Moreover, according to the second feature of the invention, even though the anodization time is extended sufficiently, the thickness of the first one conductivity type nanostructure PS layer can not be increased, whereby the promotion of xe2x80x9ctransformation of crystalline silicon to porous siliconxe2x80x9d can be made sufficiently. In the prior art, when the promotion of xe2x80x9ctransformation of crystalline silicon to porous siliconxe2x80x9d is made, xe2x80x9ctransformation of crystalline silicon to porous siliconxe2x80x9d is initiated from the top surface side, whereby the degree of the promotion of xe2x80x9ctransformation of crystalline silicon to porous siliconxe2x80x9d is low in the portion far from the top surface, so that the nanostructure having low luminescence efficiency is formed. Moreover, although there has been a disadvantage that the thickness thereof also is made thick more than required in the prior art, according to the second feature of the invention, the completed nanostructure PS layer whose luminescence efficiency is high can be formed uniformly in the direction of a thickness. Moreover, according to the prior art, since a certain thickness is required in order to make progress of xe2x80x9cthe transformation of crystalline silicon to porous siliconxe2x80x9d sufficiently, thinning is difficult. On the other hand, according to the second feature of the invention, even in the case of an extremely thin film thickness, transformation to the completed nanostructure PS layer can be performed easily and thinning also can be performed.
In the first feature, it has been mentioned that the series resistance further is reduced by transforming the nanostructure PS to a multi-layered structure. Therefore, in order to form the multi-layered nanostructure PS layer, in the second feature of the invention, the PS wafer comprises the second one conductivity type non-degenerate c-Si layer formed under the first one conductivity type degenerate c-Si layer; and further the second one conductivity type degenerate c-Si layer therebelow, and the second one conductivity type non-degenerate c-Si layer may be transformed to the second nanostructure PS layer by anodization. Furthermore, it is as a matter of course that by taking the structure that a second and a third one conductivity type non-degenerate c-Si layers and the degenerate c-Si layer are laminated alternately, the further multi-layered structure can be realized. The degenerate c-Si substrate may be used as the lowest degenerate c-Si layer.
Anodizing in the second feature of the invention may be performed by making contact the opposite conductivity type degenerate c-Si layer being positioned on the top layer of the described-above c-Si wafer with the anodization solution containing hydrogen fluoride; providing a metal electrode on a bottom surface of the c-Si wafer; and feeding a current through this electrode and the electrode provided in the anodization solution containing hydrogen fluoride.
In this case, when the one conductivity type is n-type (and the opposite conductivity type is p-type), anodizing is preferably performed while irradiating light.
According to the first and second features of the invention, the maximum luminescence efficiency can be secured, and a manufacturing yield of the semiconductor light emitting device becomes high and the productivity is improved.
Moreover, in the first and second feature of the invention, it is as a matter of course that the c-Si may be either single crystal silicon and polycrystalline silicon.
Other and further objects and features of the present invention will become obvious upon an understanding of the illustrative embodiments about to be described in connection with the accompanying drawings or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employing of the invention in practice.